test: tweak test to avoid unpreemptible loop with gccgo

This test contains a very tight loop with locking/unlocking that can
wind up as an unpreemptible when compiled with gccgo, depending on
inlining. Tweak the test slightly to avoid this problem.

Change-Id: I155fd2b4bfea961244eb6c6594c24ab03d32d41c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193619
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/test.go

@@ -1950,11 +1950,20 @@ func test27660(t *testing.T) {
 			// increase the likelihood that the race described in #27660
 			// results in corruption of ThreadSanitizer's internal state
 			// and thus an assertion failure or segfault.
+			i := 0
 			for ctx.Err() == nil {
 				j := rand.Intn(100)
 				locks[j].Lock()
 				ints[j]++
 				locks[j].Unlock()
+				// needed for gccgo, to avoid creation of an
+				// unpreemptible "fast path" in this loop. Choice
+				// of (1<<24) is somewhat arbitrary.
+				if i%(1<<24) == 0 {
+					runtime.Gosched()
+				}
+				i++
+
 			}
 		}()
 		time.Sleep(time.Millisecond)